doc: add comment about highWaterMark limit
Add a comment regarding memory limits and setting highWaterMark
See https://github.com/nodejs/node/pull/33387#issuecomment-629377778 and https://github.com/nodejs/node/pull/33387#issuecomment-629533171 @nodejs/streams
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines